The art of forming shaped articles from particulate materials is well known in the art. Classically, a desired particulate material is mixed with a binder and then formed into the desired shape, this being called the green body. The green body is then sintered to provide a fusion of the particulate material and to drive off the binder, thereby producing the desired shaped product with desired surface textures, strength, etc.
In the production of shaped objects in the manner above described, it has been found that it is necessary to remove the binder before the green body can be sintered to avoid cracking. This is a very difficult task, however the prior art has recognized this problem and has therefore attempted to remove binder from the shaped green body prior to the step of firing. Examples of such prior art are set forth in the patent of Strivens U.S. Pat. No. 2,939,199 and British Pat. Nos. 779,242 and 1,516,079. Unusual conditions such as vacuum or solvent atmospheres are required by the prior art and cracking of parts due to process irregularities remains a problem. In addition, prior art solvent extraction techniques present health hazards which are difficult and expensive to eliminate. Solvent recovery and recycling by prior art methods is costly and adds a substantial overhead burden to the process. Bubbling and cracking of the green body during binder removal are present in the prior art binder extraction systems. Both bubbling and cracking are due to the internal pressure forces that are generated by the conversion of the binder from a solid or liquid phase to a vapor phase and the subsequent expansion of the vapor bubble. The conditions under which the vapor bubble expands can be either isothermal or isotropic (adiabatic) though isothermal is usually the primary way. It appears that the reason for some of this bubbling caused in the prior art binder removal system is due to the fact that the true boiling points of liquids are dependent not only on the properties of the liquids themselves but also upon the geometry and the interfacial surface enegery between the particulate material and the binder.
The original process--The Bendix Process--is about 30-40 years old. It is produced by Dimonite Corporation and employed a plural component thermoplastic binder, paraffin, beeswax and polyethylene. The injection molded part is baked out (LTB'D) for long periods of time, sometimes running into many days, to remove the binder. Hastening the burnout schedule results in cracking, blistering, etc. The Wiech Criteria is met here by very slowly advancing temperature to allow all volatiles to evaporate without causing internal Pv energy release by vapor expansion.
Strivens recognized the limitation of the Bendix process and met the Wiech Criteria simply by increasing the cohesive work/energy of the particulate system by incorporating a high strength thermoset plastic into the system. In this way he could advance temperatures much faster than the Bendix process. Later he recognized solvent extraction and met the Wiech Criteria by multiple thermoplastics.
Next, Wiech met the Wiech Criteria by a technique that minimized the expansive work/energy by insuring that the binder was liquid and can not support sheer stress.
In reviewing these processes it has become apparent that:
(1) The preferred industrial process is one that is most amenable to continuous (vs. batch) production and
(2) Maximizes the binder removal rate within the requirements of the Wiech Criteria.
Before proceeding a generalized statement of the Wiech Criteria is:
(1) Wiech Criteria: (.DELTA.pv).sub.cohesive &gt;(.DELTA.pv).sub.disruptive
The cohesive work/energy of the particulate system must be greater than the disruptive work/energy of the binder removal processes.
The thermodynamic processes involving vapors or gases are the ones that have the greatest pv terms. Practical engines are based upon vapor/gas expansion and compression rather than solid or liquid expansion and compression. For this reason, primarily, the generation of vapors and gases internal to the molded part lead to the high (.DELTA.pv) disruptive term which causes failure of the part.
The key to the successful removal without destructive disruption of a binder from a particulate based body is to provide such removal in such a way that the internal work/energy of the escaping binder is insufficient to overcome the cohesive work/energy of the particulate material. These conditions are met in the process disclosed in the British Pat. No. 1,516,079 wherein the binder is heated to a liquid state and the binder is removed while in the liquid state slowly enough to stay within the required work/energy envelope. However, relatively long extraction times and a complex series of extraction steps is the price that one pays for meeting the successful extraction criteria.
A truly successful industrial method is one that can operate on a continuous basis from the formation of the green body through the sintering process. This has not been done in the prior art.